The present invention relates to the art of high voltage power supplies. The invention finds particular application in conjunction with high voltage power supplies (often referred to as generators in the x-ray art) for x-ray tubes and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with arc limiting in other environments.
During an electric arc discharge, such as may occasionally occur in an x-ray tube at high voltage, very high frequency currents from tens of kilo-Hertz to many mega-Hertz are generated. During the arc, the x-ray tube, housing components, or other surrounding elements may be damaged. Therefore, it becomes advantageous to limit or suppress arcing to protect those elements at risk from damage.
In the past, a number of methods have been used for limiting arcing. One such method was to insert an electrical resistance, typically in the form of fixed resistors, in the path of the output voltage. However, depending on the desired output power of the system, this could cause significant voltage loss and power dissipation during normal operation. Moreover, destructive currents were still permitted to flow during an arc.
Another method was to introduce a series inductance in the output with the intent of reducing the rate of rise of the arcing current. Reducing the rise spread the event over time which limited the current peak. While reasonably effective, this method was rather costly. The inductive elements could also require cores, flyback protective diodes, and damping resistance to catch the inductive voltage spikes that resulted from rapidly changing currents. The size and number of coils necessary to achieve the desired inductance tended to be costly. Furthermore, the resultant circuitry took up significant space and could tend to introduce failure modes of its own.
Still another method attempts to match the characteristic impedance of the high voltage transmission cable through a series terminating resistance of relatively low value, typically in the range of 50-100 ohms. This matching then reduces the resonant ringing from the parasitic reactance of the supply cable and load. However, large currents are still permitted to flow, and typically, it is implemented in only the anode end of an x-ray tube as opposed to both the anode and cathode, where the cathode filament connections and current requirements make it impractical.
The present invention contemplates new and improved arc limiting devices which overcome the above-referenced problems and others while significantly reducing the peak arc currents and absorbing much of the arc energy. At the same time, it permits the direct load current to flow to the load with minimal DC resistance, voltage, and power loss.